A low-cost system to measure low-level nitrogen dioxide (NO₂) in an atmosphere was developed. Its major component included a sensor comprising filter paper treated with 3,3'-dimethylbenzidine ( o -tolidine), a compound that reacts with NO₂ to generate color proportional to the NO₂ contacted. In addition to the system development, its utility was optimized and the characteristics of the sensor were defined. Originally developed to determine the NO₂ levels during direct-fired tobacco curing, the system may be used to determine the NO₂ levels in other situations.